1. Field of the Invention
The invention relates to a process for the recovery of lead, arising especially from the active material of spent batteries. It also relates to an electric furnace intended especially for the use of the process.
2. Art Background
Henceforth, the main industrially significant application of lead lies in its use as a material for forming batteries.
Moreover, as a result of the importance of the market for batteries, it is estimated that approximately 50% of lead production arises from the recycling of this metal present in batteries.
Generally, the spent battery consists of lead (weakly alloyed metal, in the sulphate or oxide form), plastics and sulphuric acid, the major impurity being sulphur.
The process which requires the least handling consists in charging the batteries as such into a blast furnace identical to those used for first melting and in recovering the sulphur either in the gas form (SO.sub.2), but unfortunately in the very dilute form, or, by modifying the reduction setting of the furnace, in a ferrous matte, which has many disadvantages regarding its possible storage in waste tips.
All the other processes require a prior physical grading beginning with crushing batteries in the presence of water, which makes it possible to separate the metallic lead (battery lead connections and accumulator plates), plastic and fines. The fines, for their part, are 70 to 80% composed of lead, of which especially approximately half is found in the sulphate form due to the spent nature of the battery, the remainder of this lead is mainly found in the oxide state and optionally, for a small proportion, in the metallic state. The other components of the fines, apart from lead and sulphur, are plastic particles arising from the shortcomings of the separation means, as well as a small amount of silica.
To the knowledge of the Applicant, a number of recovery processes are currently used starting from battery fines as obtained as indicated above.
A first process consists of trapping the sulphur in a sodium slag (by addition of sodium and iron). This slag, which has a low melting point (850.degree.-950.degree. C.), can be easily removed and the use of a simple rotary furnace is sufficient. Nevertheless, the sodium slag is relatively unstable which makes it difficult to store it on a long term basis in a waste tip.
A second process consists in lixiviating the battery fines with a sodium carbonate solution in order to separate quantitatively the lead carbonate from the sodium sulphate solution which is discarded. The lead carbonate is then treated in a suitable unit. Discarding or enhancing the value of the sodium sulphate is, however, difficult.
A third process consists in melting the fines in a reverberatory furnace in an oxidizing medium produce crude lead and a lead-rich slag (PbO) and in then reducing the said slag in an electric furnace.
Nevertheless, the SO.sub.2 produced during the first stage is very dilute and expensive to trap or to recover.
Other documents describe a process involving an electric reduction furnace.
Document FR-A-2,414,558 describes a process for extracting nonferrous metals contained in slags and other metallurgical by-products containing nonferrous metals in the form of compounds, in which these materials are processed in the molten state by heating them by resistance in an electric furnace containing immersed electrodes, under a layer of solid reducing agent, and by injecting a non-oxidizing gas into the molten material so as to produce efficient mixing of the said material, characterized in that the non-oxidizing gas is injected at a flow rate between 0.5 and 10 Nm.sup.3 /hour/tonne of material treated.
This process thus requires the prior production of the slags, having a low sulphur content, in another furnace and involves mixing by injection of gas.
Document FR-A-2,297,254 proposes a reduction, in one stage in an induction furnace, of a lead slurry, after having removed water therefrom, by mixing with anhydrous pulverulent carbon so as to cause dissociation of the lead compounds (lead peroxide PbO.sub.2, lead sulphate PbSO.sub.4) and then to extract the sulphur dioxide from the reaction gases. It is shown, page 3, that the amount of coke added represents from 2 to 9% with respect to the dried product; such a range of control will correspond to very different degrees of reduction and to very different levels of removal of the sulphur; this process, which consists of only a single stage, has the disadvantage of not being able to produce simultaneously a slag having a low lead content and a good yield of removal of the sulphur.
It is shown, in the description on page 2, lines 5 to 10, that this process makes it possible to increase by several times the relative SO.sub.2 content of the gases with respect to the known processes, this factor making it possible not only to remove dust from the gas in a simple way at the furnace outlet but, moreover, making it possible to separate easily the SO.sub.2 therefrom.
It is, in effect, a major objective of any process intended for reprocessing battery fines and for recovering the lead from the latter. To the knowledge of the Applicant, this process cannot be used industrially for processing the products shown while both removing the sulphur and producing a slag which is impoverished in lead.
Document U.S. Pat. No. 4,571,261 also proposes a single-stage reduction of a lead slurry by heating with a burner and vigorous stirring and corresponding evolution of sulphur dioxide.
Likewise with such a process, it is not possible to obtain simultaneously a slag having a low lead content and a good yield for removal of sulphur.
Document FR-A-2,342,345 describes a process for processing lead residues in the non-dissolved state, produced during the production of electrolytic zinc. The lead is largely in the sulphate form and is present in the residues in a proportion varying from 10 to 60%. The said process comprises the direct, simultaneous and essentially continuous addition to an electric furnace containing partially immersed electrodes
a) of the lead residues, PA1 b) of a reducing agent containing carbon, PA1 c) of fluxes for producing a slag containing FeO, CaO and SiO.sub.2 ; and PA1 of producing this SO.sub.2 gas in a concentrated form in order to facilitate its recovery therefrom, PA1 of producing a stable slag which can be stored anywhere while respecting the environment or even reemployed for construction uses in civil engineering or building, PA1 of significantly reducing the amounts of dust produced either by volatilization of the lead and of its compounds or by escapes during charging of the fines. PA1 a) melting the fines, especially from spent batteries, under not very reducing conditions in the presence of a small amount of carbon so as to separate the crude lead, on the one hand, and a lead-rich slag, on the other hand, so as to remove the sulphur in the form of sulphur dioxide and, optionally, recovery of the crude lead, PA1 b) reduction of the lead-rich slag with a suitable amount of carbon so as to separate, on the one hand, a reduced substantially, lead-free slag and the crude lead; PA1 c) recovery of the crude lead from Stages b) and optionally a). PA1 the by-products based on lead sulphate resulting from the industries for the manufacture of lead accumulators, PA1 the residues from factories for the electrolysis of zinc containing lead, zinc, iron and other metals, as well as sulphur in the form of sulphate, PA1 dust resulting from factories for the incineration of household waste.
maintaining the temperature of the layer of molten slag in the electric furnace within the range from 1000.degree.-1500.degree. C. to produce at least two separate layers of the molten phase in the said furnace, one of the said layers being a slag layer and the other of the said layers being an crude lead layer; and
the recovery from this furnace of a crude lead containing less than 1.1% of sulphur and of a slag containing not more than 4% of lead.
This process nevertheless has the following disadvantages in the case of the recovery of lead from battery fines:
The reduction setting shown, which corresponds to an oxygen potential equivalent to 4% of lead in the slag, does not make it possible in the case of battery fines to sufficiently remove sulphur in the gases, in the form of SO.sub.2, and consequently leads to the preservation of a significant amount, indeed of all, of the sulphur in the molten products in the form of a matte which is difficult to separate from the slag or, in all cases, difficult to reprocess or to store.